1. Field of Endeavor
The invention deals with the field of materials science. It relates to a nickel-base superalloy, in particular for the production of single-crystal components (SX alloy) or components with a directionally solidified microstructure (DS alloy), such as for example blades or vanes for gas turbines, but also for conventionally cast components.
2. Brief Description of the Related Art
Nickel-base superalloys of the aforementioned type are known. Single-crystal components made from these alloys have a very good material strength at high temperatures. This allows, for example, the intake temperature of gas turbines to be increased, with the result that the efficiency of the gas turbine rises.
Nickel-base superalloys for single-crystal components, as are known from U.S. Pat. No. 4,643,782, EP 0 208 645 and U.S. Pat. No. 5,270,123, for this purpose contain alloying elements which strengthen the solid solution, for example Re, W, Mo, Co, Cr, and elements which form γ′ phases, for example Al, Ta and Ti. The level of high-melting alloying elements (W, Mo, Re) in the basic matrix (austenitic γ phase) increases continuously as the temperature of load on the alloy increases. For example, standard nickel-base superalloys for single crystals contain 6-8% W, about 3-6% Re and up to 2% Mo (in % by weight). The alloys disclosed in the abovementioned documents have a high creep strength, good LCF (low cycle fatigue) and HCF (high cycle fatigue) properties and a high resistance to oxidation.
These known alloys were developed for aircraft turbines and were therefore optimized for short-term and medium-term use, i.e., the load time was designed for up to 20 000 hours. By contrast, industrial gas turbine components have to be designed for a load time of up to 75 000 hours, i.e., for long-term loading.
By way of example, after a load time of 300 hours, the alloy CMSX-4, which is known from U.S. Pat. No. 4,643,782, when it was tested for use in a gas turbine at a temperature of over 1000° C., underwent considerable coarsening of the γ′ phase, which disadvantageously leads to an increase in the creep rate of the alloy.
On account of the long-term loading of gas turbines, it is therefore necessary to improve the resistance of the known alloys to oxidation at very high temperatures.
It is known from GB 2 234 521 A that enriching nickel-base superalloys with boron or carbon during a directional solidification produces microstructures which have an equiaxed or prismatic grain structure. Carbon and boron strengthen the grain boundaries, since C and B cause the precipitation of carbides and borides at the grain boundaries, and these compounds are stable at high temperatures. Moreover, the presence of these elements in and along the grain boundaries delays the diffusion process, which is a primary cause of the grain boundary weakness. It is therefore possible to increase the misorientations (usually 6°) to 10° to 12° yet still achieve good properties of the material at high temperatures.
EP 1 359 231 B1 discloses a nickel-base superalloy which has improved casting properties and a higher resistance to oxidation than known nickel-base superalloys and is additionally suitable, for example, particularly for large gas turbine single-crystal components having a length of >80 mm. The nickel-base superalloy disclosed therein is characterized by the following chemical composition (details in % by weight): 7.7-8.3 Cr, 5.0-5.25 Co, 2.0-2.1 Mo, 7.8-8.3 W, 5.8-6.1 Ta, 4.9-5.1 Al, 1.3-1.4 Ti, 0.11-0.15 Si, 0.11-0.15 Hf, 200-750 ppm C, 50-400 ppm B, remainder nickel and production-related impurities. A preferred alloy having the composition (in % by weight): 7.7-8.3 Cr, 5.0-5.25 Co, 2.0-2.1 Mo, 7.8-8.3 W, 5.8-6.1 Ta, 4.9-5.1 Al, 1.3-1.4 Ti, 0.11-0.15 Si, 0.11-0.15 Hf, 200-300 ppm C, 50-100 ppm B, remainder nickel and production-related impurities, is outstandingly suitable for producing large single-crystal components, for example blades or vanes for gas turbines.